A transmission is a mechanism train, which consists of various series and parallel combinations so that the driven member of one mechanism is the driver for another mechanism (Ref: J E Shigley and J J Uickers, Theory of Machines and Mechanisms, Second Edition, McGraw Hill, 1995). If there is a single input to the transmission to drive the output, then the transmission is single degree of freedom. If a multitude of inputs are required to drive the outputs, then the transmission is multiple degree of freedom mechanism. Simple transmissions consist of series combinations only and are single degree of freedom. Planetary gear transmissions or trains (PGTs) are characterized by the presence of parallel combinations. A simple PGT has two degrees of freedom. In a transmissions with parallel combination, it is possible to have internal power loops existing and power flowing in such loops is called re-circulating power. In some cases re-circulating power in a transmission can exceed the input power by a large factor.
The invention being described here is useful for situations where there is no single operating point of a system. An example of such a system is a machine tool, which operates at different operating conditions of depth of cut, cutting speeds and feed rates depending on the geometry, material and processing sequence. Another example of such a system is a vehicle, which operates at different speeds and load conditions depending on the traffic, road grade, etc. Yet another example of such a system is a legged mobile system where there are two distinct phases of operation for each leg—namely, stance phase and transfer phase.
In the examples considered above, it may be useful to have multiple prime movers, each suited to some range of operating points. A combination of power from such prime movers is also envisaged in other ranges of operating points.
In the case of a machine tool spindle drive, an electric drive will operate efficiently at some range of motor speeds and torques. If during some cutting operations, the motor speed or the torque demanded of the motor or both were much different as compared to the efficient range, then the drive motor may consume large amount of input power compared to the power delivered at the spindle. Thus, it is desirable from the point of efficiency of operation that two prime movers be used to drive the spindle.
In the case of the legged walking machines, the legs supporting the weight of the machine typically move at slow speed but under substantial loads. The legs in transfer mode, on the other hand, move in the air where the speeds are high while the loads are small (see for example, Kharade, A, Issac, K. K., Amarnath, C., Seth, B. et al “Nataraj: A Six Legged Walking Robot for Nuclear Power Plants”, Proceedings of the PLANET CARRIERS 2002 Conference, Thiruvanthapuram, India, November 2002). A single motor catering to both the requirements would not always operate most efficiently.
In the case of an automobile, it is well known that the best efficiency and least pollution operations correspond to narrow ranges of operating parameters. However, the automobile has to operate in conditions that are dictated by the traffic and road conditions, coupled with vagaries of power requirements like those in parking, reversing, cruising, and idling, which often result in operation outside the desirable ranges of operating parameters. For example, efficiency is low and rate of pollution is high in automobiles at the starting conditions.
Vehicles driven only by cleaner power sources such as electric batteries have been sought as a way to overcome problem of ICE pollution. However, the requirements of travel duration, maintaining speed during grade climbing and sufficiency of acceleration for meeting performance specifications—have only been met with consequent larger sizes of batteries to power the motor. The alternatives to conventional batteries—fuel cells, photovoltaic cells, etc., are currently being explored but each has its set of problems. The problems like unavailability of hydrogen refilling stations hinder the commercial implementation of fuel cell technology. The size of solar panels is a hindrance to the use of photovoltaic cells. Another prime-mover that does not pollute and is suited to powering the wheels at low speeds can be integrated into the automobile system if a suitable hybrid transmission system is used.
Hybrid Electric Vehicles—assimilation of electric power with power of the conventional ICE—consequently, have been sought as a compromise. A Hybrid Vehicle transmission would accept two inputs, i.e., an ICE and an electrical motor, whereas the transmission gearbox of an ordinary vehicle would accept only one input, i.e., an ICE.
In HEVs, electric motor is powered from an on-board source of power, such as a battery. If a HEV has to be practically feasible, battery will have to be periodically charged. A desirable way to charge the battery is to utilize the overhauling characteristics of the load while retarding (regenerative braking). Another way to charge the battery is to utilize part of the power from the IC engine. The HEV transmission should permit driving a generator, which would make electric power available for charging the battery in both cases. This could be achieved by using a dedicated generator or possibly by using the electric motor itself as a generator. The latter has the advantage of fewer components in the system. (Ref: Tsai L. W., Schultz G., Higuchi N., A Novel Parallel Hybrid Transmission, Trans. ASME, Journal of Mechanical Design, Vol. 123, June 2001, pp. 161-168.)
HEVs come primarily in two configurations—series and parallel. Series configuration was the first attempt to circumvent the large size of batteries required for an extended travel. This is achieved by an ICE, dedicated solely for the purpose of charging batteries. The ICE working point is therefore chosen freely. Since power flows from ICE to the battery (via a generator) first and then from battery to the motor, there are extra power conversions required in this configuration. This leads to reduced system efficiency. In a parallel HEV, either of the electric motor and ICE or both can drive the vehicle. A single motor capable of doubling up as a generator also suffices. There are fewer power conversions, which help increase the vehicle efficiency. In a parallel HEV there are at least three modes of operation possible: pure electrical, pure ICE and power operation with both the sources. In some of these cases, it may be possible to charge batteries in regenerative way.
A spin-off of parallel HEV is a Power-Split Hybrid (PSH). A generator is included in this system to keep charging the battery. The drawback of this arrangement is that it is unable to transmit the entire engine torque to the output shaft and it needs two electrical machines. (Ref: Jonasson K., Analyzing Hybrid Drive System Topologies, Licentiate Thesis, Lund University, Sweden, 2002)